Ocean Ecosystems: Open Ocean

The realm of open water, called the pelagic zone, has the greatest volume and vertical range of any life zone. It includes the region above the continental shelf , called the neritic province, and the region beyond, called the oceanic province. Gradations in light, temperature, water chemistry, nutrient content, and pressure result in a diversity of environments that are filled by a large number of species.

Life is found throughout the water column (that is, top to bottom), but mostly in the photic zone, the region where sunlight makes photosynthesis possible. Organisms are also more abundant where there are more nutrients: in the neritic, where nutrients wash off the land, and in upwelling zones, where relatively cold nutrient-rich waters from the deep ocean rise to the surface. Pelagic life is dominated by plankton , mostly tiny organisms that move with water currents. Photosynthesis by phytoplankton is directly or indirectly the primary food source for all marine life. The animals, or zooplankton, that eat them may also be tiny, like krill, or they may be larger, like jellyfish, and able to make small, directed motions.

The active swimmers that inhabit the open ocean are called nekton. While the vast majority of nekton are fish and mammals, they include invertebrates, such as mollusks and crustaceans. The most productive waters in the world are upwelling zones, such as those of the west coast of South America. Here, the abundance of nutrients supports a large population of phytoplankton, which in turn is the foundation of rich fishing grounds. If upwelling stops, as happens off Peru during an El Niño event, the fish population declines; if the fishery has already been weakened by overfishing, it may collapse, as did the Peruvian anchovy fishery in the early 1970s.

The ocean has a moderating effect on world climate because water has a high ability to absorb and store heat. When prevailing winds come off an ocean the climate is milder than in locations with no oceanic influence. This is why annual temperature fluctuations are much smaller in western than in eastern coastal North America. The surface layer of the ocean is a heat reservoir that may maintain temperature anomalies for years, and alter rainfall patterns. For example, increased sea surface temperature results in increased evaporation. This increases rainfall and therefore condensation, which provides the energy to drive an El Niño event.

The enormous productivity of phytoplankton has a large effect on the atmosphere, since these organisms use carbon dioxide (CO2) and release oxygen. Also, CO2 is highly soluble in seawater and the ocean is a carbon dioxide
sink. Manipulations of oceanic chemistry have been proposed to control atmospheric levels of CO2, and possibly reduce greenhouse warming. In large regions of the ocean, phytoplankton growth is limited by lack of the trace element iron. In two experiments, small patches of the sea surface were fertilized with minute amounts of dissolved iron. This triggered a massive phytoplankton bloom: the phytoplankton growth rate doubled, its biomass increased by nearly thirty times, and its nitrate uptake increased by fourteen times. If phytoplankton populations were increased on a wide scale, phytoplankton might use more CO2. When these organisms died, some would fall to the seafloor, taking with them the carbon they had harvested from atmospheric CO2.

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Ocean Ecosystems: Hard Bottoms

The term "hard bottom" refers to the ocean region close to shore, where wave action prevents the accumulation of muddy sediment that will create a soft bottom. Plants and animals living on or in the seafloor are called benthos. Benthic epifauna reside on or attach to a rocky substrate (surface). Benthic infauna bury themselves in soft sediments or bore into the rocky bottom or shells of other animals.

Seaweeds, a kind of algae, anchor to the bottom with holdfasts. Unlike the roots of higher plants, holdfasts do not extract nutrients. Many seaweeds have pneumatocysts, gas-filled bladders that keep the photosynthetic parts of the seaweed in the photic zone, the near-surface layer of the ocean that light penetrates. All benthic animals are invertebrates. Although they inhabit all water depths, most are in the photic zone where light and nutrients are more abundant. Many benthic animals are sessile . Some are suspension feeders; they strain the fresh seawater brought in by waves, tides, or currents for plankton and other nutrients. Others wait for food to arrive.

When a small fish swims against a sea anemone's tentacles, it is stunned by poisonous nematocysts and then dragged into the anemone's central mouth. Some benthic animals can move to pursue their prey, scavenge over the bottom, or graze on seaweed-covered rocks. Starfish crawl on tube feet over a shellfish or sea urchin; they pry apart the shell with their arms, extrude their stomach into the prey's shell, and begin digestion. Many benthic animals exist for part of their life cycle as tiny planktonic larvae, drifting with the water to colonize new areas.

Benthic epifauna have enormous species diversity, reflecting the diversity in the benthic environment. In the intertidal—between the highest high and the lowest low tides—small distances bring large variations in substrate, temperature, salinity, moisture, pH level, wave action, dissolved oxygen, and food supply. Organisms at the top must contend with weather, predators from land, crashing waves, and an occasional influx of fresh water from storms. Organisms at the bottom face ocean predators and sometimes land predators, waves, weather, and drying out. For protection against predators and desiccation , the animals of the intertidal have shells (clams and barnacles) or exoskeletons (crabs and lobsters), or they appear as crusts on rocks (lichens and algae). Sea anemones huddle together to conserve moisture, and segmented worms and crabs retreat into mussel beds or rocks.
For protection against waves and tides, intertidal organisms attach to rocks with holdfasts (seaweed), cement (barnacles), threads (mussels), or arms (starfish). Snails squeeze a muscular foot tightly into a rock and retreat into their shells.

The rocky intertidal has been the site of important ecological studies. In the rocky intertidal of Washington state, the top carnivore is the starfish Pisaster ochraceus. Removal of the starfish caused a decline in the number of species from fifteen to eight, including an enormous increase in the population of the mussel Mytelus edulis, the starfish's favorite prey. Pisaster ochraceus is therefore a keystone predator in its community; by limiting the size of mussel's population, it clears out space for other species. Keystone predators are often at a high risk for extinction since they are often high in the food chain and sparsely distributed.

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Ocean Ecosystems: Soft Bottoms

Where water movements are not strong enough to wash them away, sediments coat much of the benthic environment. Soft bottoms are common along coasts, along continental margins, and in the deep sea. Plants and animals that attach to sandy or rocky surfaces are called benthic epifauna; those that bury themselves in soft sediments or bore into the rocky bottom or shells of other animals are called benthic infauna. Few benthic organisms can live in shifting sediments, as on a beach exposed to wave action, and many more are found in sediments in protected bays or estuaries.

The plants of soft bottoms are marine angiosperms, seed-bearing vascular plants with true roots. So that they can photosynthesize, benthic plants live only in the photic zone. Grasses are the only marine plants that live on soft bottoms. They catch sediments and organic matter in their roots, protecting the shoreline from erosion and providing shelter, substrate, and food for a diverse group of animals. Much organic material from these wetland ecosystems is carried offshore, where it provides nutrients for organisms living beneath the photic zone.

The animals found on the shore or in near-shore sediments live primarily on plankton and organic debris from land. Suspension feeders attach themselves to hard or sandy bottoms and strain water for food. Filter feeders are similarly attached but actively pump large amounts of water through their bodies to get food. Many benthic infauna are deposit feeders who eat sediments, extracting the organic matter trapped between the grains. Predators and scavengers on soft bottoms include starfish, snails, cephalopods, and crustaceans. Bacteria are an important protein source and play a major role in decomposition.

The benthic environment is extremely diverse in water depth, temperature, salinity, substrate type, and predation and competition. The most important factor determining the distribution of near-shore benthic infauna is grain size. Large-grained particles, such as sands, are fairly porous. They gain and lose water, gases, and organic material quickly. Filter feeders attach to sands, since smaller sediments are easily swept up by water and can clog the animals' mucus-lined filtration systems. Deposit feeders prefer to live in the top 1 to 2 centimeters of organic-rich, fine-grained mud. This is the aerobic zone, where dissolved oxygen permeates. Below the oxygenated layer is the black, oxygen-depleted, or anaerobic , zone, where only anaerobic bacteria can live fully. Anaerobic bacteria produce hydrogen sulfide, the rotten egg smell of black mud. Some animals, like clams, live in the anaerobic zone to avoid predators but extend siphons into the aerobic zone to obtain food and oxygen.

The deep sea is uniformly cold and dense, and sediment particles are small and relatively uniform in size. The number of benthic species increases from the near shore to the deep ocean but the number of individuals and total biomass decreases. All major groups of shallow water benthos have deep ocean counterparts. But shortage of food causes the deep-sea organisms to be smaller, live longer, and reproduce less frequently. Most deep-sea organisms are deposit feeders with a few conspicuous filter feeders and predators.

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